An overhead sectional door assembly commonly involves the following components:
A pair of guide tracks extending vertically up from ground level along each side of the door opening frame, the tracks curving to a horizontal position at the top of the opening; PA1 A sectional door formed of hinging panels and having rotatable rollers attached along the door side edges, whereby the rollers run along the guide tracks as the door is lifted, thereby controlling the positioning of the door. The panels hinge or bend as the door rounds the track curve from vertical to horizontal positions; PA1 A pair of lift cables, each secured to one lower corner of the door. The cables extend up and over cable drums at the top corners of the door frame; PA1 Latch means for locking the door in the lowered position; and PA1 Torsion or extension spring means positioned at the top or along the sides of the door frame and connected with the cables. The spring means function to pull the cables up over the drums, thereby raising the door when the latch means is released. PA1 a bracket is secured to the lower corner of the door; PA1 the bracket supports a horizontal shaft having a partly toothed disc at one end; PA1 the disc is positioned within the track. The disc has a smooth portion which is in contact with the track so that the disc can glide along the track when the door is in motion. Otherwise stated, the disc functions as a bottom roller for the door; PA1 an arm protrudes from the shaft--the arm has an eye for engaging the lifting cable; PA1 a torsion spring is positioned around the shaft. The spring is anchored at its inner end to a lug attached to the bracket. At its outer end the spring is connected with the shaft. PA1 a bracket for mounting to one lower corner of an overhead sectional door, the bracket having a central web and inner and outer forwardly protruding, parallel legs; PA1 a roller shaft mounted to the legs and carrying a rotatable roller on its outer end for running in the adjacent door guide track, the shaft further carrying a sheave, spaced inwardly from the roller, around which the lifting cable may extend; PA1 the bracket having an upwardly extending section carrying means for anchoring the free end of the lifting cable; PA1 a brake shaft, rotatably mounted to the legs above and forwardly of the roller shaft, the brake shaft being positioned to clear the forward end wall of the track and to be forwardly spaced therefrom; PA1 the brake shaft carrying a cam member having at least one downwardly and rearwardly directed tooth, for engaging the outer surface of the forward end wall of the track, and an arm having an eye through which the anchored end of the cable extends as it passes around the sheave; PA1 the brake shaft carrying a torsion spring secured at its outer end to spring-anchoring means attached to the shaft and at its inner end to spring-tensioning means secured to the inner leg of the bracket. PA1 the utilization of two spaced apart shafts, the rear shaft carrying the roller for controlling the positioning of the door, bracket and breaking teeth relative to the track, the forward shaft locating the cam member and teeth outside the track and ensuring that the positioning of the teeth is fixed relative to the roller, so that improved clearance is created and the teeth are better kept from contacting the track at its curve; PA1 the cam member functioning to bring the teeth into contact with the track in an angular position so that the door weight will then drive the teeth firmly into the track with a wedging biting action, when the torsion spring is released; and PA1 the utilization of a tensioning device on the brake shaft, for varying the tension of the spring to better match it with the type of lift involved and the weight of the door used.
Now, there is a potential for the cables or lifting spring means to part or fail. If the door is being raised or lowered, this can result in the door dropping and injury or damage may follow.
Safety brake mechanisms have been marketed and patented for stopping the door from falling when tension in the cable is lost. See U. S. Pat. No. 5,291,686, issued to Sears et al, and brochures made of record herewith, as examples of this prior art.
These known safety brake mechanisms typically involve the following components:
As long as the cable is taut, the arm is restrained by the cable. The spring is torqued and ready to unwind, but it is prevented from unwinding by its end connections with the lug and shaft. When the cable breaks, the arm is released, the spring rotates the shaft and disc and drives the teeth into the guide track to brake the door and prevent it from dropping.
These known safety brake mechanisms have been associated with problems which have affected acceptance in the market place.
It needs to be understood that there are three different main types of overhead door systems. The first system is referred to as a "standard lift". In this system the door goes immediately into an overhead position as soon as it begins to lift off the floor. The lift cables are initially in tension in an amount equal to the total door weight. This initial tension gradually decreases almost to zero once the door is fully open and entirely horizontal. The second system is referred to as a "hi-lift" assembly. In this case, the door travels upwardly for a pre-determined distance before it begins to travel overhead or horizontally. The lift cables are initially in tension equal to the door weight. This initial tension gradually decreases to equal that portion of the door weight which has not travelled overhead once the door is fully open. In the third system, referred to as the "vertical lift" system, the door travels upwardly without travelling overhead. Therefore the lift cables remain in tension at all times in an amount equal to the total door weight.
In other words, with the standard lift system the cable tension is greatly reduced when the door is fully open, particularly if the door is small and light, whereas the cables in the hi-lift and vertical lift systems always retain appreciable tension when the door is fully open. The known safety brakes work adequately for the hi-lift and vertical lift systems in some situations. However they are unreliable with the standard lift doors because the torsion spring is liable to unwind slightly when the cable is untensioned, causing the teeth to engage the track in a braking action. This is, of course, undesired.
Another problem can arise in situations where an electrically operated motor drives a shaft which carries drums on which the lift cables are wound or unwound. When the electric motor is first started, it has a tendency to cause some slack to arise in the lift cables, which again can undesirably cause the brake to set. This is particularly likely to occur with the standard lift doors.
It is therefore an objective of the invention to provide a safety brake mechanism wherein the tension in the torsion spring can be adjusted to optimize or match the spring to the door assembly with which it is being used, to thereby reduce the likelihood of undesired triggering of the braking action.
For purposes of this description, "outer" denotes furthest from the center of the door, "inner" denotes closest to the center of the door, "rearward" denotes toward the door surface and "forward" denotes away from the door surface.